1. Field of the Invention
The present invention relates to a dynamic DC offset canceling apparatus and a dynamic DC offset canceling method for a receiver in wireless communication, and primarily relates to a dynamic DC offset canceling apparatus and a dynamic DC offset canceling method used in the receiver having a radio section adopting the direct conversion scheme in digital mobile communication.
2. Description of Related Art
The receiver in digital mobile communication receives radio waves from a plurality of paths with different propagation times, due to the influence of multipath fading which may occur in a radio link. Therefore, delay distortion is produced in the received signal and bit error rate characteristics deteriorate. Faster symbol rates produce larger delay distortion, and so with future high-speed digital mobile communication equipment, the influence of delay distortion cannot be ignored, and cancellation of delay distortion becomes essential.
An equalizer is a typical means for correcting delay distortion. In radio communication, conventionally, equalizers are mounted on receivers that perform high-speed transmission, and, further, in recent years, equalizers are mounted on mobile telephones and the like adopting GSM (Global System for Mobile Communications), which is the European digital mobile telephone standard. Further, there is an increase in the number of receivers that improve equalization accuracy and improve bit error rate characteristics by estimating the DC offset component, which may be mainly produced in a radio RF section, using an average value of the received signal and canceling the DC offset component as preprocessing of the equalizer.
FIG. 11 is a block diagram showing the configuration of a receiver with a conventional DC offset canceling apparatus, and FIG. 12 shows the configuration of a DC offset compensating processing section in the receiver. In FIG. 11 and FIG. 12, receiver 10 with the DC offset canceling apparatus is configured with DC offset compensating processing section 11, equalizing processing section 12 and decoding processing section 13, and DC offset compensating processing section 11 is configured with average value calculating section 14 and DC offset canceling section 15. In DC offset compensating processing section 11, average value calculating section 14 estimates a DC offset component using an average value of the received signals, and DC offset canceling section 15 cancels the DC offset component.
In recent years, to reduce the circuit scale (cost) of the radio RF section, the direct conversion reception scheme is adopted instead of the heterodyne reception scheme which converts a received frequency to an intermediate frequency. However, to accompany the spread of the direct conversion reception scheme, in the narrow band digital modulation scheme used in GSM and the like, the received signal frequency is equal to a local frequency, and so the DC offset component remains in a detection output of the receiver (baseband received signal), and, in particular in GSM, DC offset (hereinafter “dynamic DC offset”) increases rapidly in the middle of the burst, and, as a result, DC offset may become difficult to cancel by estimating an average value of the received signals, which may result in deterioration of bit error rate characteristics. Further, in GSM, known codes (training sequence) for acquiring synchronization and assisting reception are embedded in the middle of the burst, and dynamic DC offset in the middle of the burst deteriorates bit error rate characteristics significantly.
To cancel dynamic DC offset, with the conventional DC offset canceling algorithm, digital baseband signals are mapped on an IQ diagram, and the I and Q coordinates of the center point are determined by two-dimensional fitting of a geometric figure using a subset of signal values. It is possible to learn changes with dynamic DC offset from changes of the center point (see, for example, Patent Document 1: Japanese Patent Application Laid-Open No. 2000-278335).
However, a problem with such a conventional DC offset canceling apparatus is that calculation for determining the center point for an arbitrary consecutive subset of digital baseband signals increases the amount of processing.